The present invention relates to a polyester film for decorative sheets. The decorative sheet used herein generally means a decorative material (surface material) attached to the surface of a substrate for various products such as furnitures, building components or household appliances and provided on a surface thereof with a picture-printed layer. Whereas, the decorative plate used herein generally means a building material (surface material) comprising at least a film layer and a picture-printed layer which are successively laminated on the surface of a substrate, and usually dealt as an independent commercial product. Accordingly, the film layer and the picture-printed layer laminated on the surface of the substrate of the decorative plate are constituents of the decorative sheet.
As a sheet material (film) interposed between the substrate and the picture-printed layer of the decorative plate or used in the decorative sheet, polyvinylchloride resin sheets have been most generally used. However, in the case where the polyvinylchloride resin sheet is used, there arise such problems that a plasticizer blended in the sheet is migrated to an adjacent adhesive layer, resulting in deteriorated adhesion between the sheet and a substrate, and that the polyvinylchloride resin sheet suffers from elongation or shrinkage upon heating due to low thermal dimensional stability thereof, thereby causing wrinkles thereon. Further, it has been reported that the polyvinylchloride sheet generates a chlorine gas upon burning, thereby causing sources of acid rain or dioxin. In consequence, it has been strongly demanded that decorative plates or decorative sheets should be produced without using the polyvinylchloride resin sheet from a standpoint of environmental protection.
On the other hand, since the decorative plates and the decorative sheets are used as a surface material, picture patterns having a high design value are usually formed on a surface thereof. Therefore, it is extremely important to control color tones of the decorative plate or the decorative sheets in order to obtain appropriate contrasts between the picture patterns or delicate contrasts of shade and shadow thereon.
Meanwhile, since the decorative plates and the decorative sheets are used as surface materials, it is inevitable that these plates and sheets come into contact with tip ends of cleaners, fingernails or toenails, etc., so that the surfaces thereof sometimes become damaged. Especially when the surfaces of the decorative plates or the decorative sheets undergo severe contact, scratch or abrasion, much larger damages are caused thereon, so that a surface or an inside of a underlying film layer tends to be exposed to the surfaces of the decorative plates or the decorative sheets. In such a case, when a color tone of the film layer is different from that of a printed layer provided on the surface of the film layer, the damaged portions are remarkably contrasted with surrounding portions due to the difference in color tone therebetween, thereby causing severe damage to a design value of the surfaces of the decorative plates or the decorative sheets.
Also, in the production of polyester films, there have been caused film scraps trimmed or separated off, e.g., from edge portions of biaxially-stretched films because these edge portions are unsuitable as commercial products. The film scraps are formed into pellets and then re-used as reclaimed polyester. The blending percentage of the reclaimed polyester to virgin polyester is about 3 to about 70% by weight though it is varied depending upon types of films to be produced or an aimed production efficiency. Accordingly, in the case where the above-mentioned reclaimed polyester is used for the production of polyester films, it becomes possible to obtain inexpensive decorative plates or decorative sheets using the polyester film as an alternate material of polyvinylchloride.
However, the reclaimed polyester obtained from the production process of color pigment-containing polyester films which are used as films for decorative plates or decorative sheets, suffers from considerable change in color tone during reclaiming process therefor before being formed into pellets. Therefore, when such a reclaimed polyester is mixed with virgin polyester, the color tone of the resultant polyester film is affected in a large extent.
As a result of the present inventors' earnest studies, it has been found that in use for a decorative sheet comprising at least a picture-printed layer and a film layer which will be successively laminated on a substrate thereof, by adjusting a color difference between the film layer and the picture-printed layer to a specified value, or by constituting the film layer from at least one layer and at least one other layer and adjusting a color difference between the layer and the other layer to a specified value, the above problems can be solved. The present invention has been attained by the above findings.